1. Field of the Invention
The present invention relates to a method for recording information, represented by the arrangement of codes of three or more kinds, on a recording medium having a track, a method for reproducing the recorded information from said medium, and an apparatus therefor.
2. Related Background Art
The optical memory industry has shown growth in recent years, and the recording media has been expanded from the optical video disks and compact disks for reproduction only to the add-on optical disks utilizing thin metal films or recording dye materials. The research and development at present are directed toward rewritable optical disks utilizing a magnetooptical recording method or phase transition recording method. Also, the application is being expanded from consumer products to the external memories of computers. In such optical memory apparatus, important are control technology for causing a small light spot to follow a desired track, and recording/reproducing technology enabling high density recording and high-speed information reading, and various proposals have been made regarding such control technology and recording/reproducing technology, in relation to the progress in the optical disks. As an example, an optical information recording/reproducing method for high density recording and high-speed information reading, and an apparatus therefor are disclosed in Japanese Patent Application Laid-open No. 2-172039. The details of this patent application will be explained in the following, with reference to the attached drawings.
At first, FIG. 1 shows information pits formed on an optical disk. Said pits are formed as recesses or projections on an information track, with a depth or a height equal to 1/4 of the wavelength of the information reading laser beam. Said information pits are recorded in seven different manners, according to the number of unit pits and the positions in the transverse direction of the track. Consequently the information is recorded by multi-value recording of 7 values, corresponding to codes 0-6. More specifically, the center of the unit pit can assume three positions in the transverse direction of the track, namely a position on the center of the track and positions displaced from said center by about 1/2 of the width of the unit pit. A code 0 is represented by two units pits recorded at left and right with respect to the center of the track. A code 1 is represented by shifting the center position of the above-mentioned information pits to the center position at left, while a code 2 is represented by shifting to the center position at right. A code 3 is represented by a unit pit recorded at the track center, while a code 4 is represented by a unit pit at the center position at left, and a code 5 is represented by a unit pit at the center position at right. A code 6 is represented by the absence of a recorded pit.
An apparatus for such multi-value recording will be explained in the following, with reference to FIG. 2, in which shown are a code conversion circuit 102, a laser driving circuit 103, a laser 104, an actuator 105, an optical disk 108, and a spindle motor 109. Recording data 101 are converted, in the code conversion circuit 102, into information of the combination of the center position of the pit in the transverse direction of the track and of the pit width (number of unit pits). Said information is interpreted as the position of a unit pit in the transverse direction of the track and the presence or absence of said unit pit. In the information recording, a same track is scanned by a light beam spot by the number of positions of said unit pit with altered tracking positions, and unit pits are recorded in the scanning at each unit pit position, according to the information indicating the position of the unit pit in the transverse direction and the presence or absence of the unit pit. For example, in the 7-value recording shown in FIG. 1, the same track is scanned three times as the unit pit can assume three positions. Therefore, in each scanning, the actuator 105 is given a unit pit position signal 107 indicating an offset of 0 or .+-., whereby the scanning position is set at the track center or displaced therefrom in the + or - direction by about 1/2 of the width of the unit pit. Then the laser driving circuit 103 is given a signal 106 indicating the presence or absence of a unit pit in this scanning position, whereby the intensity of the laser beam from the laser 104 is varied to form information pits as shown in FIG. 1, on a recording surface of the optical disk 108 maintained in rotation by the spindle motor 109.
FIG. 3 shows an apparatus for reproducing the information of the thus formed 7-value recording, wherein shown are a laser 110, an objective lens 111, a two-divided photodetector 113, amplifiers 114, 115, a subtractor 116, an adder 117, comparators 118a-118d, and an encoder 119. A difference signal D' obtained from the subtractor 116 is supplied to the comparators 118a, 118b and is compared with reference voltages Va, Vb. Also, a sum signal A' obtained from the adder 111 is supplied to the comparators 118c, 118d and is compared with reference voltages Vc, Vd. Output signals from said comparators are converted by the encoder 119 into reproduced information 120.
In the following, the function of the information reproducing apparatus shown in FIG. 3 will be explained in further detail, with reference to a timing chart shown in FIG. 4. Referring to FIG. 4, the sum signal A' indicates the presence or absence of a pit and a width thereof. Said signal becomes lower than the reference voltage Vc in the presence of a pit, and becomes lower than the reference voltage Vd when the pit width is larger than the width of the unit pit, as in the code 0, 1 or 2. A binary signal (c) can be obtained by slicing the sum signal at Vc and giving a value "1" to a signal level equal to or higher than Vc. Also a binary signal (d) can be obtained by slicing said signal at Vd and giving a value "1" to a signal level equal to or higher than Vd. Also, the difference signal D' indicates the pit position. If the pit is symmetrical with respect to the track center or is totally absent, the difference signal D' is positioned between the reference voltages Va and Vb. The difference signal D' becomes higher than the reference voltage Va when the center of the pit in the transverse direction of the track is displaced upwards from the track center (code 2 or 5), and said signal D' becomes lower than the reference Voltage Vb when the center of pit is displaced downwards (code 1 or 4). Thus, a binary signal (a) can be obtained by slicing said difference signal at Va and giving a value "1" to a signal level equal to or higher than Va, and a binary signal (b) can be obtained by slicing at Vb and giving a value "1" to a signal level equal to or higher than Vb. The reproduced information can be obtained by supplying the binary signals (a), (b), (c) and (d) to the encoder and detecting a code corresponding to the combination of said binary signals to decode the code.
The above-explained recording method, and the apparatus for information recording and reproduction provide the advantages of enabling multi-value recording, thereby drastically increasing the recording density and also enabling high-speed reading of the information recorded with a high density. However, in the above-explained apparatus, the number of multiple values is limited because the information recording is based on the combination of information pits in the transverse direction of the track. Although said number can be increased, through the increase in the number of combinations, by an increase in the number of center positions of the unit pit in said transverse direction, the recording speed becomes inevitably lower as the number of light beam scannings accordingly increases at the information recording, and this fact is also a limiting factor of the number of multiple values.